Deprecated grains: white corn ear rot

White ear rot is a disease that causes great damage to corn crops, as it directly affects the commercial product, that is, the grains. This disease is caused by two types of fungi, Stenocarpella maydis [synonym of Diplodia maydis] e Stenocarpella macrospora [synonym of Diplodia macrospora] which can cause infection and result in “burnt grains”.

The grains infected by these fungi are brown in color (Figure 1) and when the frequency and severity of the attack are high, the formation of a whitish layer is observed between the rows of grains on the ear, responsible for the name “white ear rot”. ” (Figure 2). The development of the disease has the potential to affect part or all of the ear, and may begin at the base or the tip of the ear. (Chick et al.

White ear rot affects production in several aspects, such as, for example, a drop in productivity and characteristics of the harvested grains, in addition to the production of mycotoxins that can influence the economic value of the grain and a reduction in the nutritional quality of the feed (Molin & Valentini, 1999). A greater intensity of ear rot generally leads to an increase in the incidence of burnt grains, which are undesirable for sale, due to the requirement of agribusinesses to acquire quality raw materials. At the time of delivery of the final product, the amount paid may be reduced due to the presence of burnt grains (Reis et al.

White rot can be controlled in several ways. The most common and efficient methods are the use of genetic, chemical and cultural control. In the literature it is possible to find some works on these control methods.

Figure 1 - Burnt grains	Figure 2 - Ear with white rot

Figure 1 - Burnt grains

Figure 2 - Ear with white rot

genetic control

There are currently a large number of hybrids available on the market, many presented as carrying genetic resistance to various diseases. In 2010 Gralak et al evaluated the behavior of six commercial hybrids, divided into two groups: tolerant (2B604, P30R50 and Formula) and susceptible (2B710, DKB390 and P30K64) regarding S. maydis. The authors found a difference in the susceptibility of hybrids to the fungus that causes white ear rot, with hybrids 2B604, P30R50, Formula and P30K64 (Table 1) showing the lowest percentages of burned grains. The authors inferred that the correct choice of hybrid is capable of reducing agronomic losses in grain productivity.

Table 1 - Production and percentage of burned grains of six commercial hybrids subjected to inoculation with the fungus Stenocarpella maydis

Hybrid	Burnt grain (%)	Productivity (kg ha-1)
2B604	27,66a	10,556b
P30R50	35,33a	11,433a
Formula	15,00a	10,000b
2B710	53,33b	9,477 b
DKB390	50,00b	10,992a
P30K64	22,33a	12,162a
CV%	33,94	8,34
Adapted from Gralak et al, 2010. Behavior of Commercial Corn Hybrids Inoculated in the Field with the Fungus Stenocarpella maydis

Hybrid

Burnt grain (%)

Productivity (kg ha^-1)

2B604

27,66a

10,556b

P30R50

35,33a

11,433a

Formula

15,00a

10,000b

2B710

53,33b

9,477 b

DKB390

50,00b

10,992a

P30K64

22,33a

12,162a

CV%

33,94

8,34

Adapted from Gralak et al, 2010. Behavior of Commercial Corn Hybrids Inoculated in the Field with the Fungus Stenocarpella maydis

Mendes(2010), testing resistance of ten hybrids in relation to resistance to the fungus S. maydis, found that the NB7210 hybrid also had the highest percentage of burned grains, followed by the DKB390 and 2B710 hybrids, with values ​​exceeding 22% (Table 2). According to the author, the fact that there were hybrids with high percentages of burned grains, when inoculated with S. maydis, allows us to infer that there is a difference between the hybrids and that they have different behavior for each of the fungi that cause white rot. For the fungus S. macrospora hybrid 2B710 had the highest incidence of burned grains, with a value of 27%, thus belonging to the group of hybrids considered susceptible. The hybrid that obtained the lowest percentage was 2A525, with 4,39%, thus belonging to the group considered resistant to rot-causing fungi.

TABLE 2 - Average results of percentage of burned grains in ten corn hybrids, depending on inoculation with fungi S. maydis e S. macrospora and control treatment (T – without inoculation) depending on the agricultural harvests (2006/07 and 2007/08), in a conventional cultivation system

HYBRIDS	2006/07 Harvest	2007/2008 Harvest
S. maydis	S. macrospora	Witness	S. maydis	S. macrospora	Witness
AG 6018	1,60 aB	1,81 aB	3,37 aA	11,43cA	8,29cA	2,15 aA
AG 8021	3,17 aB	1,72 aB	2,92 aA	8,46 bA	4,85 bA	4,71 bA
DKB 199	2,84 aA	3,98 aA	2,81 aA	5,87 aA	1,78 aA	3,03 aA
2 A 525	5,10 bB	3,08 aB	3,33 aA	0,84cA	6,08cA	3,10 aA
NB 7215	7,74 bB	2,97 aB	3,94 aA	26,33 eA	22,56 eA	5,67 bA
DKB 350	4,60 bB	3,88 aB	2,83 aB	19,65 dA	17,11 dA	6,62 bA
DKB 390	15,08 cB	3,47 aB	2,09 aB	19,07 dA	31,07 gA	6,77 bA
P 30F53	2,33 aB	2,63 aB	2,11 aB	5,78 aA	7,23cA	9,14cA
2/710/XNUMX B XNUMX	4,81 bB	3,37 aB	2,92 aA	24,89 eA	26,75 eA	4,83 bA
NB 7210	7,08 bB	2,69 aB	3,49 aB	25,12 eA	39,37hA	13,53 dA
Means followed by the same lowercase letters in the column and capital letters in the row belong to the same grouping using the Scott Knott test, at a significance level of 5%. Adapted from Mendes et al, 2010. Behavior of corn hybrids inoculated with the fungi that cause the burned grain complex.

HYBRIDS

2006/07 Harvest

2007/2008 Harvest

S. maydis

S. macrospora

Witness

S. maydis

S. macrospora

Witness

AG 6018

1,60 aB

1,81 aB

3,37 aA

11,43cA

8,29cA

2,15 aA

AG 8021

3,17 aB

1,72 aB

2,92 aA

8,46 bA

4,85 bA

4,71 bA

DKB 199

2,84 aA

3,98 aA

2,81 aA

5,87 aA

1,78 aA

3,03 aA

2 A 525

5,10 bB

3,08 aB

3,33 aA

0,84cA

6,08cA

3,10 aA

NB 7215

7,74 bB

2,97 aB

3,94 aA

26,33 eA

22,56 eA

5,67 bA

DKB 350

4,60 bB

3,88 aB

2,83 aB

19,65 dA

17,11 dA

6,62 bA

DKB 390

15,08 cB

3,47 aB

2,09 aB

19,07 dA

31,07 gA

6,77 bA

P 30F53

2,33 aB

2,63 aB

2,11 aB

5,78 aA

7,23cA

9,14cA

2/710/XNUMX B XNUMX

4,81 bB

3,37 aB

2,92 aA

24,89 eA

26,75 eA

4,83 bA

NB 7210

7,08 bB

2,69 aB

3,49 aB

25,12 eA

39,37hA

13,53 dA

Means followed by the same lowercase letters in the column and capital letters in the row belong to the same grouping using the Scott Knott test, at a significance level of 5%.

Adapted from Mendes et al, 2010. Behavior of corn hybrids inoculated with the fungi that cause the burned grain complex.

It is important to highlight that in all integrated management programs, genetic control must be the first to be adopted. The difference in commercial materials available on the market, with different degrees of resistance to white rot, provides an advantage to the producer in managing this disease.

chemical control

 The use of chemical control is often necessary, given the losses resulting from this disease. Oak et al, 2004, testing the influence of different doses of thiabendazole on the incidence of S. maydis, found that treatment with fungicide significantly reduced the incidence of S. maydis, from approximately 70% in untreated seeds to less than 10% in treated seeds (Figure 3A).

In the same work, in the germination result, there was an increase in the percentage of germinated seeds with the increase in doses used in the tests (Figures 3B and 3C). The seedling germination speed index was influenced by the dose of fungicide used (Figure 3D). Seeds treated with thiabendazole, at an intermediate dose, showed faster emergence.

FIGURE 3 - Incidence of Stenocarpella maydis (A), germination (%) under ideal temperature conditions (B), and in the cold test (C) and germination speed index (GVI) (D) of corn seeds (zea mays) treated with different doses of thiabendazole

Source: Carvalho et al, 2004. Relationship of Corn Seed Size and Fungicide Doses in the Control of Stenocarpella maydis

Currently on the website of the Ministry of Agriculture and Supply (Mapa) there are only two products registered for the control of S. maydis, both with the same active ingredient and consequently the same mode of action. This fact deserves attention due to the impossibility of product rotation, which fatally increases the risk of resistance of isolates to this fungicide.

 

Cultural control

Although little publicized and often ignored by farmers, cultural practices are highly effective in controlling plant diseases, especially when traditional control methods do not work. These practices are relatively simple and low-cost to implement.

In work to verify the effect of plant density on the incidence of burned grains, in rotation and monoculture systems, Trento et al observed that as the density of plants increases, the incidence of fungi also increases both in crops carried out in monoculture systems and in crop rotation (Figure 4). The plant population can also be considered an important factor in the incidence of ear rot, the high density of plants, high levels of nitrogen and low levels of potassium as factors that indicate greater susceptibility to pathogens, in addition to creating a microclimate favorable to development of pathogens.

Another observation is that in crops grown in a monoculture system the incidence of burned grains was higher when compared to crop rotation. The presence of infected ears remaining on the soil surface from one agricultural year to another, as occurred in monoculture, serves as a source of inoculum contributing to the increased incidence of burned grains in this system.

FIGURE 4 - Effect of corn plant density (zea mays) on the incidence of burned grains in rotation and monoculture systems

Source: Trento et al, 2002. Effect of crop rotation, monoculture and plant density on the incidence of burned grains in corn.

Another important cultural measure, which influences the incidence of white rot and consequently burnt grains, is the moment of harvest. In evaluating corn grains from hybrids XL 344 and XL 2124 regarding the incidence of burned grains, Santin et al observed an increase in the two hybrids cultivated on the two soil covers (oats and turnips) until the fourth evaluation (Figure 5). In the following evaluations, a reduction in the incidence of burned grains was detected. This is possibly explained by the fact that the increase in symptoms of grain colonization by fungi occurred while the grains had moisture levels above 18%. Below this grain moisture content, infection and colonization cease to be symptomatic.

FIGURE 5 - Incidence of burned grains (GA) in corn grains of hybrids XL 212 and Na = turnip, evaluated in seven collections at intervals of nine days

Source: Santin et al, 2004. Effect of delaying the corn harvest on the incidence of burned grains and pathogenic fungi.

Controlling white rot is possible in several ways, such as choosing resistant material, using pesticides and relatively simple cultural practices, such as crop rotation, ideal planting density and harvesting at the appropriate time. However, to carry out effective and sustainable control over time, the greatest number of measures together as possible must be used, avoiding adopting just one certain tactic because it is easier or less expensive.

Click here to read the article in the online edition of Cultivar Grandes Culturas issue 185.